Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric tracks of a magnetic disc medium, the actual information being stored in the forward magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle, while the information is accessed by read/write heads generally located on a pivoting arm which moves radially over the surface of the rotating disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
During operation, the discs are rotated at very high speeds within an enclosed housing using an electric motor generally located inside the hub or below the discs. Such known spindle motors typically have had a spindle mounted by two ball bearing systems to a motor shaft disposed in the center of the hub. The bearings are spaced apart, with one located near the top of the spindle and the other spaced a distance away. These bearings allow support of the spindle or hub about the shaft, and allow for a stable rotational relative movement between the shaft and the spindle or hub while maintaining accurate alignment of the spindle and shaft. The bearings themselves are normally lubricated by highly refined grease or oil.
The conventional ball bearing system described above is prone to several shortcomings. First is the problem of vibration generated by the balls rolling on the bearing raceways. This is one of the conditions that generally guarantee physical contact between raceways and balls, in spite of the lubrication provided by the bearing oil or grease. Hence, bearing balls running on the generally even and smooth, but microscopically uneven and rough raceways, transmit the rough surface structure as well as their imperfections in sphericity in the vibration of the rotating disc. This vibration results in misalignment between the data tracks and the read/write transducer. This source of vibration limits the data track density and the overall performance of the disc drive system. Vibration results in misalignment between the data tracks and the read/write transducer. Vibration limits therefore the data track density and the overall performance of the disc drive system.
Further, mechanical bearings are not always scalable to smaller dimensions. This is a significant drawback, since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit.
As an alternative to conventional ball bearing spindle systems, much effort has been focused on developing a fluid dynamic bearing. In these types of systems lubricating fluid, either gas or liquid, functions as the actual bearing surface between a stationary shaft supported from the base of the housing, and the rotating spindle or hub. Liquid lubricants comprising oil, more complex fluids, or other lubricants have been utilized in such fluid dynamic bearings. The reason for the popularity of the use of such fluids is the elimination of the vibrations caused by mechanical contact in a ball bearing system, and the ability to scale the fluid dynamic bearing to smaller and smaller sizes.
It is important to the long term life and reliable operation of a fluid dynamic bearing that the fluid level be maintained over a long term. However, the fluid may in many instances be jarred out of the bearing gap by shock or vibrations; further, variations in operating an environmentals temperatures may cause some evaporation of the fluid over the long term.
Most current designs of fluid dynamic bearing motors employ a capillary seal at least at one end of the bearing to maintain the fluid in the bearing. This seal which is typically established by having one of the two walls of the fluid dynamic bearing gap diverge from the other, thereby establishing a miniscus across the end of the fluid column in the gap, the seal utilizes capillary attractive force to retain the oil or other fluid within the gap; maintaining the fluid during non-operating shock and vibration events.
Because the gap progressively widens before terminating in the miniscus within the capillary seal, the capillary seal can also serve as an oil reservoir to provide for oil lost due to evaporation. However, the two functions, providing a seal and providing a reservoir are somewhat conflicting. A large reservoir requires a wide capillary seal gap; but the wider the gap, the lower the oil retention capability. As the height of a fluid dynamic bearing motor is reduced in order to use such motors in smaller profile disc drives, less space is available for the capillary seal. Therefore, it is desirable to make the seal as short as possible so as to maximize the height available for the bearings to support the relative rotation of the shaft and the sleeve. As the capillary seal becomes shorter, its volume decreases, reducing the oil available to make up for that lost due to evaporation or shock. A desirable seal would have a relatively large reservoir volume, and high shock resistance. If the oil volume were sufficiently large, lower viscosity (implying a higher evaporation rate) oil could be used, lowering the power loss due to viscous friction. If properly chosen, the lower viscosity oil would have a flatter viscosity versus temperature curve, resulting in a more efficient fluid bearing over the temperature range. Therefore, a large volume, higher shock resistant fluid reservoir which uses minimal axial space is a highly desirable design objective.